Moog Sonic Six portable synthesizer

Apr 21, 2018

Moog’s portable 1970’s suit case synthesizer

Moog produced many instruments during their various incarnations throughout the second half of the 20th century and continue to design and manufacture synthesizers to this very day. There were many ups and downs for the pioneering engineer Robert Moog, he was designing circuits and building instruments that had never been seen before at the same time as learning to run a company and a manufacturing production line and as if that wasn’t enough, he and his small team had to convince artists and music stores that these strange electronic devices were practical and useful musical instruments that were worth buying, learning to use and to be early adoptors and to risk taking on stage and into the studio.

The Sonic Six has it’s own story and it was almost not made at all. This isn’t the place to repeat the story of the synth that Dr Moog didn’t want to make, it was born from a synthesizer called the Sonic Five which was designed by one of his ex-employees for another company and it took the purchasing of RA Moog Music by MuSonics to bring Dr Moog into the Sonic Six story. The story has been well documented already and a search on the internet will reveal more information, Gordon Reid has a very good article on his site on the history of the Sonic Six.

A suitcase design that worked

The iconic Moog logo

The Sonic Six is a striking piece of industrial design. It’s a beautiful looking synthesizer, and to 21st century eyes it has a very pleasing retro appearance that is made even better by the fact that it looks that way without looking particularly dated and because Moog/MuSonics managed to combine a great look with practicality. It folds into a compact case and you can put it on a table or desk, open it like a brief case, connect a power lead and you are playing immediately. No headphones or PA required, there is an amp and speaker built right in. It is very tactile, it’s interesting to explore and use and it’s ergonomics are great. One thing that I always notice with the Sonic Six is how small it looks when closed and how big it looks when open. For a synth lover opening a Sonic Six is like opening a little blue world to play around inside of and there’s lots to do in there. When you are done it closes up and disappears until the next time you want to use it.

Classic industrial design of the period

The layout inside the case is practical and spacious, it was clearly designed for educational purposes and song writers – there is a large moulded recess in the centre directly behind the keyboard that is big enough to hold a small tuner, pencils, erasers and any other bits and pieces that a student or song writer might need. Everything is easily available from the front of the instrument – The only thing that is not inside is the power input and fuse which are on the left side of the synth case. No searching around the back of the instrument for jacks or switches, everything is right on the front – headphone and line out jacks, The external input jack and gain control, accessory power connector, power switch, speaker and monitor bypass switches (a nice touch) and some CV/pedal inputs. Good descriptions of the controls and their layout are already available online so lets open the casework and take a look inside.

A look inside – The lower case section

Power and audio switches

Power and audio switches rear view

External audio input and interface connctors

Ext. input section rear view

Beginning with the bottom section of the case we remove 8 screws and this allows us to lift the moulded cover from around the keyboard. This reveals a nicely laid out tray with a metal chassis that provieds a mounting point for the key action, transformer and bender module. Our first glimpse inside is a favourable one – Quality components and a nice mechanical build and layout. Everything is nicely accessible for servicing. The action is a Pratt Read unit and in this case the silicon mouldings that hold the J-wires are the domed type rather than the adjustable flat type with multiple holes.

The mains filter capacitors beneath the volume slider board

Something interesting is immediately seen – The two sliders for the master volume and glissando controls appear to be the same type of sliders that ARP used in their 2600 synth. They are smooth and reliable. in front of the two sliders is the horiontally aligned bend wheel which has a good, strong return spring providing a definite re-centering action. The bender pot is a good quality closed frame part well protected against dust. The rest of the bender module is quite simple – you can see the 4 mains rectifier diodes to the right of the volume slider, between the sliders is a resistor and a couple of other diodes (and an IC and more diodes depending on the version) to provide a centre “dead band” for the pitch wheel to ensure that it cannot pull the synth off pitch if it doesn’t return to centre perfectly. Beneath the circuit board are two hefty silver coloured filtering capacitors with solid screw terminals. In the image below you can see the black mains transformer that is mounted behind the action and it is multi-voltage and can be switched for use in different countries by a selector slider beside the mains input socket.

The mains transformer behind the keyboard action

The images below show two views of the bender module that is beside the keyboard with a close-up showing the rectifier diodes (which are new parts that were recently replaced in this synthesizer). It’s not uncommon for these diodes to fail in Minimoog Model D’s and in the Sonic Six. There isn’t much else in the lower section, a couple of terminal blocks and some wiring and that’s it.

The bender module

The 4 power rectifier diodes

Behind the front panel

To see the rest of the Sonic Six internals we need to remove the front panel. It’s held in place by 10 phillips head screws and can sometimes catch a little on the foam surround on the inside of the case so removing it takes some attention. Placing some padding such as a folded towel on top of the keyboard allows the front panel to be swung down on it’s face onto the bottom of the case on top of the keyboard. If you intend to un-mount the internal circuit boards then before you unscrew the front panel you will need to remove the knobs, slider caps and the little caps over the slide switches to allow the circuit boards to come free of the front panel. you only have to remove the knobs from one side of the front panel if you only intend to access one of the two boards.

The two main boards with the front panel removed

In the image above you can see the two circuit boards disconnected from the front panel, the rear of which is visible at the bottom of the photo. The boards are held in place by small pins projecting from the rear of the front panel and they can be seen in the photo above. Small spring steel clips latch onto these pins and hold the boards in place, these make for quick removal and re-installation of the boards. In the image below you can see a close-up of one of these clips. All electrical connections to, from and between the two circuit boards are made via a long, thin joiner circuit board that links the two main boards together and connects them to a pair of multicore cables that run down to the bottom case section. This board is held in place by some tiny screws and it’s the first place to check if your Sonic Six is behaving erratically – Removing it and treating it with some contact cleaner and re-seating it can clear up any problems caused by tarnished contact pins. It’s a clever design indeed, very neat and it would have made assembly at the factory quite easy.

A spring steel board retaining clip

Inter-board connector

Inter-board connector close-up

Inter-board connector underside

Next we have the two circuit boards that contain all of the synthesizer circuitry. They are of high quality with large ground planes covering bare places on the boards, they contain good quality componets and they are well laid out. Because of the large size of the front panel and the relatively simple circuits there is plenty of board real-estate and the designer therefore had plenty of space to lay out the circuits and the ground planes help to take care of any noise induced into the long trace runs.

Board 1 (the left hand side board)

Board 1

The image above shows what Moog called “board 1” which is the board on the left hand side of the synth. Because the boards are so large you can actually locate and see the circuitry associated with most of the synthesizer’s elements simply by looking at the sliders – looking at the left side of the board in the above photo you can see the pots and sliders of each of the LFO’s and the circuitry for each of the two LFO’s is laid out underneath each of the respective clusters of controls.

The power supply

On the right hand side of the image at the top you can see two power transistors in metal cans, this section of the board is the power supply area – it produces the stable +/- 15V symmetrical power rails used by all of the Sonic Six’s circuits. Older analogue synth circuits use many operational amplifier chips and they need a power supply that can swing above and below zero volts, hence the need for a positive and a negative supply. Right between the two power transistors you can see the shaft of the VCO balance pot. The two long IC’s near the transistors are two dedicated voltage regulator devices, they are LM723 IC’s and they can be found in the power supplies of many synths of the era.

The VCO’s

Below the power supply on the bottom right side of the board are the two VCO’s that are marked as “tone oscillators” on the front panel. Looking at the circuitry you can see the left-right symmetry of the componet layout, oscillator A on the left and oscillator B on the right. The VCO’s are a triangle core design known as a “dual slope integrator” that uses the venerable and rare UA726 heated matched transistor array in it’s linear to exponential converter. The triangle waveform is then fed into a wave shaper section that converts it into the sawtooth and square waves as well and these comprise the three selectable audio waveforms available on the front panel.

The LFO’s

The final circuits on this board are the two modulating oscillators – A pair of LFO’s referred to as waveform generators X and Y on the front panel. They are dual slope integrator type oscillators just like the VCO’s but set up to operate at much lower frequencies. This LFO section is a powerful part of the Sonic Six – The two LFO’s can be routed to different parts of the synth individually and they also be mixed to produce more complex modulation waveforms and their levels can be controlled separately or together. these LFO’s are made even mpre potent because you they are voltage controllable from the contour generator so you can have LFO rates change singly or together with the rise and fall of the attack and decay slopes.

Board 2 (the Right hand side board)

Board 2

The image above shows the right hand side board – “board 2” in the service manual. This board contains the Sonic Six’s signal processing circuits – The filter, VCA, the envelope (contour) generator and the audio mixers along with the keyboard circuitry, the noise generator, the ring modulator and the power amplifier that drives the inbuilt speaker.

The VCF

Starting with the VCF we find the classic Moog transistor ladder low-pass filter design. I have only seen transistor type filters in the Sonic Sixes that I have come across, some of the earlier units may have had a diode filter instead. The image below shows the filter and you can see from the layout of the parts where it gets it’s name. It certainly has the classic Moog filter sound and it is very stable – one of the notable things about the Sonic Six’s filter is that it can be tuned to track the keyboard surprisingly closely – Once set up correctly you can set the resonance slider to full so that the filter becomes a sine wave oscillator and then use the cutoff frequency slider to tune the filter note so that it is at correct pitch to be played by the keyboard and it becomes a rather haunting “third oscillator”. While most mono synths can have their filters set up this way they are usually not very accurate with their scaling, the Sonic Six is notable for having a filter that is especially useful as a chromatic tone source.

The heart of any Moog – The transistor ladder filter

The VCA’s

Next we have the VCA’s – there are actually two of them – straight forward designs using a CA3080 transconductance op-amp that is a variable gain device. One VCA allows control for the final output volume of the synth under control of the volume pot and the external level input jack. This can be a little confusing for those use to other synths as there is also another VCA in the Sonic Six that is marked on the front panel as the “articulator” which is controlled exclusively by the contour (envelope) generator. In other synths the articulator is what most musicians would think of as the VCA. In the Sonic Six the CA38080’s are the old metal can type (see the image below).

A CA3080 op-amp – The VCA

Fairchild 796HC op-amp

The contour generator

The contour generator is a simple AD design – Attack and decay are all the control that you get and it is the thing that limits the Sonic Six and is regarded and the only severe weakness of the instrument. It’s a simple circuit using a timing capacitor and a comparator and can be switched between an AD or an ASR response. The decay and release are both set by the one control.

The keyboard and glissando circuits

The keyboard circuit is quite complex, it generates a precise DC current source along with a 8kHz AC signal that is rectified to produce a gate and two trigger signals. The circuitry is complicated because it has to produce two triggers for the duophonic functionality and therefore it has to generate two correct pitch CV’s from the keyboard resistor chain and it’s associated sample and hold circuit where a capacitor is charged up each time that a key is played. This capacitor acts as a “voltage memory”. This is necessary because the synth has to be able to remember what the last key you played was when you release a key. If this circuit wasn’t there the synth’s pitch would drift upwards when you release a key during the release phase of the envelope generator. To add to the complexity the glissando circuit is also a part of the keyboard circuitry.

The ring modulator

The ring modulator circuit is straightforward and similar to many other synthesizers. It’s not a classic discrete ring mod using a ring of diodes and transformers, instead it uses a balanced modulator IC. Two AC signals are input to the chip and from these it generates an output via a summing amplifier which comprises the sum and difference of the two input signal frequencies.

The noise generator

Like most vintage synthsizers the Sonic Six uses the inherent noise generated by a transistor set up with a reverse biased emitter-base junction. it happens that certain semiconductor junctions produce a useful white noise under certain circumstances. It’s a selected transistor with only two of it’s three terminals connected. This noise is very, very low level and is therefore amplified substantially and then filtered to provide an additional pink noise output.

The power amplifier

Finally we have the integrated power amplifier IC that drives the small monitor speaker located on the RHS of the front panel. It’s a large module that is screwed to the circuit board and it uses the metal on the circuit board as a heat sink to remove some of the heat from the chip when it’s working hard. If you look around the botton edge of the module in the photo below you can see some white heat transfer paste applied at the factory leaking out. It’s an AP4153P made by Mitsubishi.

The power amplifier IC that drives the speaker

While the Sonic Six is somewhat limited by it’s single two stage envelope generator it makes up for that somewhat by giving the musician some uncommon features compared to most other mono-synths and of course it has THAT sound. It’s a vintage Moog and it’s unmistakable. The keybard is a full 4 octaves allowing for easier leads and solos, the filter tracks the keyboard beautifully, the pitch wheel has a strong centering spring, you can use pedals to control pitch, filter cutoff and volume, The voltage controlled LFO’s can do complex modulations and speed changes. Given that these LFO’s can run very fast and that they can be speed modulated by the envelope generator and fed to the ring modulator as a modulation source the Sonic Six surpasses most other synths when it comes to sound design. It can make sounds that would otherwise require a modular system.

The Sonic Six is very stable and very playable, the VCO’s are excellent and hold tune well so it’s eminently useable as a performance instrument. The VCO’s have plenty of modulation possibilities, oscillator B can be easily tuned to a non-chromatic scale and oscillator A can be switched to a different note priority to oscillator B and then the synth becomes duophonic! There is a noise generator, two signal mixers and a nice selection of trigger sources available.

High quality components and excellent construction

The internal speaker

High quality switches and pots

Heavy duty switching dual pots

Early metal can IC

40 years old and still going strong

Old school components

Buying guide, servicing and reliability

If you do have a Sonic Six it will be 40 to 45 years old. Think about that. An electronic device that old was so well made that it’s still working fine after all this time. Assuming that the synth hasn’t been abused it will still be going strong. It will have almost certainly had a few of parts replaced in almost a half a century, many of the parts inside are still readily available. It’s staggering to realise that some of the oldest vintage synths are running well and with care may well last another 50 years plus. Some of them, perhaps many of them will still very likely be still working at their 100th birthday if the last 45 years are an indication and if people set aside parts kits for them and the mechanical parts are used with care. Many of the parts are still being made but they are gradually ending production, the LM301 op-amp being an example of a common synth chip that has recently been announced as no longer manufactured. If you put together a spares kit with a few critical parts then your synth will be well covered for the decades to come.

Servicing the Sonic Six is not difficult. Accessability is good, the boards are high quality and the service manual is readily available. Updating some of the power supply components is a good insurance plan if you happen to have the synth open for servicing to prevent any over-voltage failures – New LM723’s are still available and the driver transistors for the negative rail power transistor are something that might be worth replacing with new parts, I have seen them fail. Rectifier diodes are another failure point and checking the condition of the main filter capacitors is a good idea. Don’t replace those capacitors out-of hand, they are good quality parts, test them and see if they are in spec rather than replace them needlessly.

The Sonic Six is relatively rare, if you are looking to purchase one they are easy to inspect. The most common issue is missing or cracked slider caps and slide switch covers and for some reason they sometimes seem to be sporting incorrect knobs on the waveform generators. Chech the condition of the case and look for cracks. If the cosmetics are good then play all of the keys to see if they are smooth and making contact. The Sonic Six is tricky to figure out initially so if you are going to check one out it’s worth reading the owners manual before you set off to see it.

Dual mixable voltage controlled LFO’s

The built in speaker is actually quite useful. All you need is a power outlet and you’re good to go